Avian influenza A (H5N1) virus infection results in ~60% mortality in patients who present respiratory abnormalities from dyspnea and pulmonary inflammation during the first 6 days post-infection to respiratory failure with hypoxemia, leading to death several days later. The mechanisms underlying the respiratory failure responsible for the infection-induced death are unclear and no effective vaccine/treatment is available. Animal studies have focused on immunology and virology of the infection without studying respiratory pathophysiology. Additionally, these studies showed that upon pulmonary infection, the lethal H5N1 viruses, differing from nonlethal ones, initially invaded (2-3 days post-infection) the vagus nerve and then brain, resulting in death 8 days post-infection, pointing to a possible vagal involvement in H5N1 pathology. Pulmonary sensory fibers traveling within the vagus nerve are composed primarily of bronchopulmonary C-fibers (PCFs). Stimulation of PCFs peripherally triggers dyspnea and pulmonary inflammation and centrally induces depressed hypoxic and hypercapnic ventilatory responses (dHVR and dHCVR). These peripheral and central effects are achieved by PCFs releasing SP into the lungs and the middle region of the nucleus tractus solitarius (mNTS) to act on local neurokinin 1 receptor (NK1R), respectively. Because dHVR and dHCVR are responsible for generating respiratory failure, we recently tested these chemoreflexes at the early stage of the viral infection. Our preliminary data showed that HK483 (a lethal H5N1 strain) but not HK486 virus (a nonlethal one) led to remarkable dHVR and dHCVR 2-3 days post-infection without viremia and killed the mice 8 days post-infection. Interestingly, this death was absent in PCF-degenerated or SP-knockout mice. Therefore, in this project, we will first characterize the HK483 virus-induced cardiorespiratory disorders by measuring cardiorespiratory activities, pulmonary changes, and chemoreflexes in mice over the infection period and correlate the disorders to respiratory failure (death), thereby building a bas for further mechanistic studies. Second, we will define that HK483 virus invades PCFs to increase their activity and sensitivity and that PCF degeneration diminishes or prevents the virus-induced respiratory disorders (death). Third, we will reveal that HK483 virus promotes PCF-dependent SP release into the lungs and mNTS to upregulate NK1R expression in PCFs and mNTS neurons receiving PCF inputs. Moreover, the effects of systemic or peripheral blockade of NK1Rs and selective lesion of mNTS NK1R neurons on the HK483 virus-induced cardiorespiratory disorders and death will be determined. In this project, electrophysiological, biochemical, pharmacological, and immunocytochemical approaches will be used. Our predicted results as described above will: 1) form a novel neurovirological concept that lethal H5N1 virus invades PCFs to induce their morphological and functional changes; 2) gain new insight into the mechanisms underlying the pathogenesis of the lethal viral infection-induced respiratory failure; and 3) catalyze the development of preventive strategies and pharmacological therapies to protect against respiratory failure and death.